1992 Volkswagen (VW) Jetta Diesel TDI Overheat Coolant Leak Issue

You said you had the problem even when you ran with no therostat in the car, and at the time I suspected there had to be another underlying problem. so I don't think your thermostat is the problem but with the high temps you've been running, it certainly is a good idea to replace it.

Rad fan shouldn't be an issue as you previously stated that this over heating happens at high speeds, which moves air through your rad faster than the fan could anyway. If the temperature is only raising when the car is in slow speed or stopped then it is possible that your thermoswitch (not to be confused with thermostat) which controls the rad fan may also be damaged, they are not expensive and you can get different tempature levels to match your thermostat. Another thing to look at is the relay that attaches to this setup as well (at least it is there on the '90)

The water pump is this vehicle (if it indeed has the proper water pump) should have the medal impeller, and as long as the bearing is good, the only other thing that could go wrong with it would the seal to leak (it's almost sick how simple the damn thing really is).

If I were you, I would consider having the whole coolent system pressure flushed, or at the least a do-it-yourself flush, to see what kinda crap comes out.

I read somewhere that some cars can overheat with no thermostat becasue the water moves too fast to absorb enough heat. I do not know if that is true or not. I gues you could also have cavitation if the velocity was too fast.

Yes, this is true, it used to happen on our circle track race cars. But the thing was the RPMs were high (much higher than that diesel would be doing).

Putting in a thermostat would always solve those overheating problems (but I think a restrictor plate would have done just as well...)

I think some of the other suggestions (water pump failure, hose collapsing, etc.) are good guesses.